The present invention relates to metal alloys, and, more particularly, to an ultra high carbon steel suitable for superplastic processing.
Superplastic forming has emerged as an important manufacturing technique, because in many cases parts may be formed to essentially their final shape in a single step. Consequently, material costs and costs of secondary processing such as machining may be significantly reduced.
Superplastic behavior is ordinarily found in metals having fine grain sizes at elevated temperatures, and is marked by a high sensitivity of the stress to strain rate during deformation. This high sensitivity prevents the growth of mechanical instabilities during processing and results in the attainment of unusually high elongations before failure, sometimes exceeding 1000 percent. The high strain rate sensitivity and high elongations to failure in turn permit the use of specialized superplastic processing techniques.
To prepare an alloy for a superplastic forming operation, the alloy must first be processed to a fine grain structure. Although in some instances superplasticity is not related to grain size, in most instances a finer grain size results in increased superplastic strain rate for any selected stress level. Most alloys must therefore first be processed to a fine grain size which is stable when the alloy is reheated for superplastic forming. If the fine grain size is not sufficiently stabilized, the grains may coarsen so much during the superplastic forming operation that the superplastic property is lost before forming may be completed, and the forming operation fails. Thus, stabilization of fine grain structures is a key to improving superplastic formability.
Most of the commercial-scale applications of superplastic forming have utilized titanium, nickel, and aluminum alloys of interest in the aerospace industry. Iron-based superplastic alloys have also been developed, including, for example, the ultra high carbon steel disclosed in U.S. Pat. No. 3,951,697. This patent relates to a process for preparing a hypereutectoid steel having a fine grain size and an array of fine iron carbides to stabilize the fine grain size during subsequent superplastic processing. The superplastic forming is then accomplished just below the eutectoid (or A.sub.1) temperature of about 725.degree. C., since the steel does not exhibit the desirable superplastic property below about 600.degree. C. or above about 750.degree. C.
While the ultra high carbon superplastic steel represents a significant advance in the art, problems remain in its economic application on a widespread industrial scale. When the steel is heated to the range of superplasticity, the fine iron carbides tend to coarsen, with the result that the fine grains also grow to larger sizes. Since a fine grain size is required for superplasticity, the growth of the grains may result in the loss of the superplastic property, even though the steel is heated to the appropriate temperature range. The superplastic forming operation must be completed before the grain size grows too large. In some cases, the processing cannot be completed before the grains coarsen so much that the superplasticity is lost, thereby making the superplastic forming operation commercially impractical.
An important consequence of the increase in grain size during heating and superplastic processing is a reduction in the allowable superplastic forming strain rate. Studies and calculations have shown that an increase in grain size from about 0.4 microns to about 2 microns can be excpected to reduce the superplastic strain rate at constant stress by about a factor of 100. Since the strain rate is essentially the reciprocal of the forming time, it may be seen that grain size coarsening is expected to increase drastically the time required to form a part. And, during the lengthened forming time, even further coarsening occurs.
A further problem in the forming of iron-based superplastic alloys results from their inherently greater strength as compared with titanium-based or aluminum-based alloys. Because the iron-based alloys such as ultra high carbon steels are stronger, their superplastic forming requires larger, more powerful, and more costly forming equipment. It is conceivable that the iron-based alloys could be made inherently weak to allow their forming or smaller equipment, but this approach would also result in an undesirable reduction in the room-temperature strength of the furnished part.
Consequently, there has been a need for an improved iron-based alloy having a more stable microstructure to retain the as-processed fine grain size for longer times. Desirably, such an iron-based alloy would permit superplastic forming at high strain rates and low stresses, without sacrificing the room-temperature properties. The high strain rates and low stresses would allow the use of small equipment and increase production rates, thereby improving the economics of the superplastic forming operation. The present invention fulfills this need, and further provides related advantages.